111111111111111111111111111111111111111111111111111111111111111111111111111
`US005627799A
`US005627799A
`Patent Number:
`[11] Patent Number:
`[11]
`Date of Patent:
`[45] Date of Patent:
`[45]
`
`5,627,799
`5,627,799
`May 6, 1997
`May 6,1997
`
`United States Patent [19]
`United States Patent [19]
`Hoshuyama
`Hoshuyama
`
`[54] BEAMFORMER USING COEFFICIENT
`[54] BEAMFORMER USING COEFFICIENT
`RESTRAINED ADAPTIVE FILTERS FOR
`RESTRAINED ADAPTIVE FILTERS FOR
`DETECTING INTERFERENCE SIGNALS
`DETECTING INTERFERENCE SIGNALS
`
`[75]
`[75]
`
`Osamu Hoshuyama, Tokyo, Japan
`Inventor:
`Inventor: Osamu Hoshuyama, Tokyo, Japan
`
`Assignee: NEC Corporation, Tokyo. Japan
`[73]
`[73] Assignee: NEC Corporation, Tokyo, Japan
`
`[21] Appl. No.: 523,059
`[21] Appl. No.: 523,059
`[22] Filed:
`Sep. 1, 1995
`[22] Filed:
`Sep. 1, 1995
`[30]
`Foreign Application Priority Data
`[30]
`Foreign Application Priority Data
`
`[JP]
`[JP]
`
`Japan .................................... 6-208635
`Japan .................................. .. 6-208635
`
`Sep. 1, 1994
`Sep. 1, 1994
`Int. Cl.6
`[51]
`...................................................... GOIS 15/00
`[51] Int. Cl.6 .................................................... .. G01S 15/00
`[52] U.S. Cl ........................... 367/121; 367/901; 367/119;
`[52] US Cl. ........................ .. 367/121; 367/901; 367/119;
`367/905; 381194
`367/905; 381/94
`[58] Field of Search .............................. 367/12, 119, 121,
`[58] Field of Search ............................ .. 367/12, 119, 121,
`367/123, 129, 901. 905, 103; 128/661.01;
`367/123, 129, 901, 905, 103; l28/661.01;
`381/94
`381194
`
`[56]
`[56]
`
`References Cited
`References Cited
`
`U.S. PATENT DOCUMENTS
`U.S. PATENT DOCUMENTS
`
`3,763,490 10/1973 Hadley et aI. .......................... 3421375
`3,763,490 10/1973 Hadley et al. ........................ .. 342/375
`9/1990 Zurek et aI. ........................... 381194.1
`4,956,867
`4,956,867
`9/1990 Zurek et al. ......................... .. 381/941
`
`OTHER PUBLICATIONS
`OTHER PUBLICATIONS
`
`L. Gri?iths et aL, “An Alternative Approach to Linearly
`L. Griffiths et al., "An Alternative Approach to Linearly
`Constrained Adaptive Beamforming”, IEEE Transactions
`Constrained Adaptive Beamforming", IEEE Transactions
`on Antennas and Propagation, vol. AP-30, No.1, Jan. 1982,
`on Antennas and Propagation, vol. AP-30, No. 1, Jan. 1982,
`pp.27-34.
`pp. 27-34.
`‘
`S. Nordholm et al., "The Board-Band Wiener Solution for
`S. Nordholm et al., ‘The Board-Band Wiener Solution for
`Griffiths-Jim Beamformers", IEEE Transactions on Signal
`Gn'?iths-Jim Beamformers”, IEEE Transactions on Signal
`Processing, vol. 40, No.2, Feb. 1992, pp. 474-479.
`Processing, vol. 40, No. 2, Feb. 1992, pp. 474-479.
`
`I. Claesson et al., “A Spatial Filtering Approach to Robust
`I. Claesson et al., "A Spatial Filtering Approach to Robust
`Adaptive Beaming", IEEE Transactions on Antennas and
`Adaptive Beaming”, IEEE Transactions on Antennas and
`Propagation, vol. 40, No. 9, Sep. 1992, pp. 1093-1096.
`Propagation, vol. 40, No.9, Sep. 1992, pp. 1093-1096.
`“Processing Signals Carried By Propagating Waves”. Mul
`"Processing Signals Carried By Propagating Waves", Mul(cid:173)
`tidimensional Digital Signal Processing, Prentice-Hall,
`tidimensional Digital Signal Processing, Prentice-Hall,
`Inc., pp. 289-315.
`Inc., pp. 289-315.
`M.M. Goodwin et al., "Constant Bearnwidth Beamforming",
`M.M. Goodwin et al., “Constant Beamwidth Beamforming”,
`Proceedings of International Conference on Acoustics,
`Proceedings of International Conference on Acoustics,
`Speech and Signal Processing 93, pp. I-169-I-l72.
`Speech and Signal Processing 93, pp. 1-169",;[-172.
`
`ABSTRACT
`ABSTRACT
`
`Primary Examiner—Ian I. Lobo
`Primary Examiner-Ian J. Lobo
`Attomey, Agent; or Firm-Sughrue, Mion, Zinn, Macpeak &
`Attorney, Agent, or Firm-Sughrue, Mion, Zinn, Macpeak &
`Seas
`Seas
`[57]
`[57]
`In an adaptive array beamformer, a spatial beamforming
`In an adaptive array beamformer, a spatial beamforming
`filter is connected to a sensor array for respectively filtering
`?lter is connected to a sensor array for respectively ?ltering
`and summing array signals to produce a first filter output
`and summing array signals to produce a ?rst ?lter output
`containing a target signal that arrives in a specified direction.
`containing a target signal that arrives in a speci?ed direction.
`First adaptive ?lters provide transversal-?ltering the ?rst
`First adaptive filters provide transversal-filtering the first
`?lter output to produce a second ?lter output not containing
`filter output to produce a second filter output not containing
`the target signal, using a ?rst error signal by restraining their
`the target signal, using a first error signal by restraining their
`tap weight coe?icients. The array signals are further coupled
`tap weight coefficients. The array signals are further coupled
`to subtractors. Each subtractor detects a difference between
`to subtractors. Each subtractor detects a difference between
`the second ?lter output of the corresponding ?rst adaptive
`the second filter output of the corresponding first adaptive
`filter and the corresponding sensor signal to derive the first
`?lter and the corresponding sensor signal to derive the ?rst
`error signal. Second adaptive ?lters provide transversal
`error signal. Second adaptive filters provide transversal(cid:173)
`?ltering the ?rst error signals of the subtractors to produce
`filtering the first error signals of the subtractors to produce
`third ?lter outputs, using a second error signal, by restrain
`third filter outputs, using a second error signal, by restrain(cid:173)
`ing their tap weight coe?icients. The third ?lter outputs are
`ing their tap weight coefficients. The third filter outputs are
`summed and subtracted from the first filter output to proquce
`surmned and subtracted from the ?rst ?lter output to produce
`an output of the beamformer, which is supplied as the second
`an output of the beamformer, which is supplied as the second
`error signal to the second adaptive filters
`error signal to the second adaptive ?lters
`
`10 Claims, 11 Drawing Sheets
`10 Claims, 11 Drawing Sheets
`
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`RTL923_1023-0001
`
`

`
`u.s. Patent
`US. Patent
`
`May 6, 1997
`May 6,1997
`
`Sheet 1 of 11
`Sheet 1 of 11
`
`5,627,799
`5,627,799
`
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`LAF = Leaky Adaptive Filter
`
`RTL923_1023-0002
`
`

`
`u.s. Patent
`US. Patent
`
`May 6, 1997
`May 6,1997
`
`Sheet 2 0f 11
`Sheet 2 of 11
`
`5,627,799
`5,627,799
`
`I
`
`I
`
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`
`FIG. 2 PRIOR ART
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`
`u.s. Patent
`US. Patent
`
`May 6,1997
`May 6, 1997
`
`Sheet 3 of 11
`
`5,627,799
`
`530
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`RTL923_1023-0004
`
`

`
`u.s. Patent
`US. Patent
`
`May 6, 1997
`May 6,1997
`
`Sheet 4 0f 11
`Sheet 4 of 11
`
`5,627,799
`5,627,799
`
`FIG. 4
`FIG. 4
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`
`

`
`u.s. Patent
`
`May 6,1997
`
`Sheet 5 of 11
`
`5,627,799
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`
`RTL923_1023-0006
`
`

`
`u.s. Patent
`US. Patent
`
`May 6, 1997
`May 6,1997
`
`Sheet 6 0f 11
`Sheet 6 of 11
`
`5,627,799
`5,627,799
`
`FIG. 6
`FIG. 6
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`RTL923_1023-0007
`
`

`
`u.s. Patent
`
`May 6,1997
`
`Sheet 7 of 11
`
`5,627,799
`
`FIG. 7
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`
`RTL923_1023-0008
`
`

`
`u.s. Patent
`
`May 6, 1997
`
`Sheet 8 of 11
`
`5,627,799
`
`FIG. 8
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`RTL923_1023-0009
`
`

`
`u.s. Patent
`US. Patent
`
`May 6, 1997
`May 6,1997
`
`Sheet 9 0f 11
`Sheet 9 of 11
`
`5,627,799
`5,627,799
`
`FIG. 9
`FIG. 9
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`RTL923_1023-0010
`
`

`
`u.s. Patent
`US. Patent
`
`May 6, 1997
`May 6,1997
`
`Sheet 10 of 11
`Sheet 10 0f 11
`
`5,627,799
`5,627,799
`
`5
`
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`CCAF = Coefficient-Constrained Adaptive Filter
`CCAF = Coefficient-Constrained Adaptive Filter
`NCAF = Norm-Constrained Adaptive Filter
`NCAF = Norm-Constrained Adaptive Filter
`
`RTL923_1023-0011
`
`

`
`u.s. Patent
`US. Patent
`
`May 6, 1997
`May 6,1997
`
`Sheet 11 of 11
`Sheet 11 0f 11
`
`5,627,799
`5,627,799
`
`FIG. 12
`FIG. 12
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`RTL923_1023-0012
`
`

`
`5,627,799
`5,627,799
`
`2
`2
`canceling capability. The diiference between the assumed
`canceling capability. The difference between the assumed
`direction and the actual arrival direction of the target signal,
`direction and the actual arrival direction of the target signal,
`or a look-direction error, is of another concern because it
`or a look-direction error, is of another concern because it
`degrades the target signal, or a look-direction error, is of
`degrades the target signal, or a look-direction error, is of
`another concern because it degrades the target signal. In
`another concern because it degrades the target signal. In
`order to compensate for this shortcoming, the spatial high
`order to compensate for this shortcoming, the spatial high(cid:173)
`pass filter bank of the prior art needs as many spatial
`pass ?lter bank of the prior art needs as many spatial
`highpass filters as is necessary to provide a wide range of
`highpass ?lters as is necessary to provide a wide range of
`angles to reject the target signal to prevent its leakage into
`angles to reject the target signal to prevent its leakage into
`10 the interference path of the bearnformer.
`the interference path of the beamformer.
`
`1
`1
`BEAMFORMER USING COEFFICIENT
`BEAMFORMER USING COEFFICIENT
`RESTRAINED ADAPTIVE FILTERS FOR
`RESTRAINED ADAYfIVE FILTERS FOR
`DETECTING INTERFERENCE SIGNALS
`DETECTING INTERFERENCE SIGNALS
`
`BACKGROUND OF THE INVENTION
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`1. Field of the Invention
`The present invention relates generally to interference
`The present invention relates generally to interference
`cancelers, and more particularly to a generalized sidelobe
`cancelers. and more particularly to a generalized sidelobe
`canceler, or adaptive beamformer for an array of sensors
`canceler, or adaptive beamformer for an array of sensors
`such as microphones and the like.
`such as microphones and the like.
`SUMMARY OF THE INVENTION
`SUMMARY OF THE INVENTION
`2. Description of the Related Art
`2. Description of the Related Art
`It is therefore an object of the present invention to provide
`It is therefore an object of the present invention to provide
`It is known that wideband signals propagating across an
`It is known that wideband signals propagating across an
`an adaptive array beamformer with a reduced number of
`an adaptive array beamformer with a reduced number of
`array of sensors in directions that are different than the beam
`array of sensors in directions that are dilferent than the beam
`sensors while allowing a look-direction error.
`steering direction of the array suffer a distortion that is 15 sensors while allowing a look-direction error.
`steering direction of the array su?er a distortion that is
`According to the present invention, there is provided an
`similar to lowpass filtering.
`According to the present invention, there is provided an
`similar to lowpass ?ltering.
`adaptive array beamformer comprising an array of spatially
`According to a prior art microphone array, signals
`adaptive array beamformer comprising an array of spatially
`According to a prior art microphone array, signals
`distributed sensors, and a spatial beamforming ?lter con
`detected by an array of microphones are lowpass filtered and
`distributed sensors, and a spatial beamforming filter con-
`detected by an array of microphones are lowpass ?ltered and
`nected to the sensors for respectively ?ltering output signals
`summed together to detect a target signal that arrives in a
`nected to the sensors for respectively filtering output signals
`summed together to detect a target signal that arrives in a
`of the sensors and summing the ?ltered output signals to
`particular direction. The adaptive microphone array beam- 20 of the sensors and summing the filtered output signals to
`particular direction. The adaptive microphone array beam
`produce a ?rst ?lter output containing a target signal arriving
`former is one form of the generalized sidelobe canceler as
`produce a first filter output containing a target signal arriving
`former is one form of the generalized sidelobe canceler as
`at the array in a speci?ed direction. A plurality of ?rst
`at the array in a specified direction. A plurality of first
`described in an article "An alternative Approach to Linearly
`described in an article “An alternative Approach to Linearly
`adaptive ?lters are provided, each having a tapped-delay line
`Constrained Adaptive Beamforming", Lloyd J. Griffiths and
`adaptive filters are provided, each having a tapped-delay line
`Constrained Adaptive Bearnforming”, Lloyd J. Grif?ths and
`connected to receive the ?rst ?lter output, a coe?icient
`Charles W. Jim, the IEEE Transactions on Antenna and
`connected to receive the first filter output, a coefficient
`Charles W. Jim, the IEEE Transactions on Antenna and
`update circuit for producing tap weight coe?icients indicat
`Propagation, Vol. AP-30, No.1, January 1982, pages 27-34. 25 update circuit for producing tap weight coefficients indicat-
`Propagation, Vol. AP-30, No. 1, January 1982, pages 27-34.
`ing correlations between tap signals from the tapped-delay
`As described in an article 'The Broad-Band Wiener Solution
`ing correlations between tap signals from the tapped-delay
`As described in an article ‘The Broad-Band Wiener Solution
`line and a ?rst error signal applied thereto, a plurality of
`for Griffiths-Jim Beamformers", S. Nordholm, 1 Claesson
`line and a first error signal applied thereto, a plurality of
`for Gri?iths-Jim Beamformers”, S. Nordholm, I. Claesson
`multipliers for weighting the tap signals with the
`and P. Eriksson, the IEEE Transactions on signal Processing,
`multipliers for weighting the tap signals with the
`and P. Eriksson, the IEEE Transactions on signal Processing,
`coefficients, respectively, and means for summing the
`Vol. 40, No.2, February 1992, pages 474-478 (hereinafter
`coefficients, respectively, and means for summing the
`Vol. 40, No. 2, February 1992, pages 474-478 (hereinafter
`weighted tap signals to produce a second ?lter output not
`referred to as Document 1), the generalized sidelobe can- 30 weighted tap signals to produce a second filter output not
`referred to as Document 1), the generalized sidelobe can
`containing the target signal. The coe?icient update means
`celer comprises, a spatial lowpass filter connected to an
`containing the target signal. The coefficient update means
`celer comprises, a spatial lowpass ?lter connected to an
`includes restraining means for preventing the coe?icients
`array of microphones for filtering signals from the array and
`includes restraining means for preventing the coefficients
`array of microphones for ?ltering signals from the array and
`from increasing inde?nitely. A plurality of ?rst subtractors
`summing the filtered signals so that only the desired signal
`from increasing indefinitely. A plurality of first subtractors
`surmning the ?ltered signals so that only the desired signal
`are provided, each detecting a difference between a corre
`is contained in the summed signal. A plurality of spatial
`are provided, each detecting a difference between a corre-
`is contained in the summed signal. A plurality of spatial
`sponding sensor signal and the second ?lter output of the
`highpass filters are provided to form a spatial highpass filter 35 sponding sensor signal and the second filter output of the
`highpass ?lters are provided to form a spatial highpass ?lter
`corresponding ?rst adaptive ?lter and supplying the ditfer
`bank. Each spatial highpass filer is connected to a selected
`corresponding first adaptive filter and supplying the differ-
`bank. Each spatial highpass ?ler is connected to a selected
`ence to the coe?icient update circuit of the corresponding
`ence to the coefficient update circuit of the corresponding
`pair of microphones for filtering and summing the sensor
`pair of microphones for ?ltering and summing the sensor
`?rst adaptive ?lter as the ?rst error signal. A plurality of
`first adaptive filter as the first error signal. A plurality of
`signals to detect the interference signals. A plurality of
`signals to detect the interference signals. A plurality of
`second adaptive ?lters are provided, each having a tapped
`adaptive filters are provided for using the interference sig-
`second adaptive filters are provided, each having a tapped-
`adaptive ?lters are provided for using the interference sig
`delay line connected to receive the error signal from a
`nals as reference signals to detect those components having 40 delay line connected to receive the error signal from a
`nals as reference signals to detect those components having
`corresponding one of the ?rst subtractors, a coe?icient
`high correlation with the interference signals contained in
`corresponding one of the first subtractors, a coefficient
`high correlation with the interference signals contained in
`update circuit for producing tap weight coe?icients indicat
`the detected target signal.
`update circuit for producing tap weight coefficients indicat-
`the detected target signal.
`ing correlations between tap signals from the tapped-delay
`Since the spatial highpass filters of Document 1 are of
`ing correlations between tap signals from the tapped-delay
`Since the spatial highpass ?lters of Document 1 are of
`line and a second error signal applied thereto, a multiply
`nonadaptive type and each uses two microphone outputs, the
`line and a second error signal applied thereto, a multiply-
`nonadaptive type and each uses two microphone outputs, the
`and-surn circuit for weighting the tap signals with the
`45
`range of signals which must be rejected is very narrow. As 45 and-sum circuit for weighting the tap signals with the
`range of signals which must be rejected is very narrow. As
`coe?icients respectively and summing the weighted tap
`a result, a slight departure from the intended direction causes
`coefficients respectively and summing the weighted tap
`aresult, a slight departure from the intended direction causes
`signals to produce a third ?lter output. The coef?cient update
`a leakage of the desired signal into the interference path of
`signals to produce a third filter output. The coefficient update
`a leakage of the desired signal into the interference path of
`circuit includes restraining means for preventing the coef
`the beamformer.
`circuit includes restraining means for preventing the coef(cid:173)
`the beamformer.
`?cients from increasing inde?nitely. An adder is provided
`ficients from increasing indefinitely. An adder is provided
`To overcome the prior art shortcoming, a proposal has
`To overcome the prior art shortcoming, a proposal has
`50 for summing the third filter outputs from the second adaptive
`for summing the third ?lter outputs from the second adaptive
`50
`been made to implement a spatial highpass filter for receiv(cid:173)
`been made to implement a spatial highpass ?lter for receiv
`filters. A second subtractor detects a difference between the
`?lters. A second subtractor detects a dilference between the
`ing more than two microphone outputs as described in an
`ing more than two microphone outputs as described in an
`first filter output and the output of the adder and supplying
`?rst ?lter output and the output of the adder and supplying
`article “A Spatial Filtering Approach to Robust Adaptive
`article "A Spatial Filtering Approach to Robust Adaptive
`the difference to the coefficient update circuit of the second
`the diiference to the coe?icient update circuit of the second
`Beaming", 1 Claesson et al, the IEEE Transactions on
`Beaming”, I. Claesson et al, the IEEE Transactions on
`adaptive filters as the second error signal.
`adaptive ?lters as the second error signal.
`Antennas and Propagation, Vol. 40, No. 9, September 1992,
`Antennas and Propagation, Vol. 40, No.9, September 1992, 55
`In a preferred embodiment, a second spatial beamforming
`55
`In a preferred embodiment, a second spatial beamforming
`pages 1093 to 1096 (hereinafter referred to as Document 2).
`pages 1093 to 1096 (hereinafter referred to as Document 2).
`filter is connected to the sensors for respectively filtering
`?lter is connected to the sensors for respectively ?ltering
`According to Document 2, each of the highpass filters that
`According to Document 2, each of the highpass ?lters that
`output signals of the sensors and summing the filtered output
`output signals of the sensors and summing the ?ltered output
`comprise the spatial highpass filter broadens the range of
`comprise the spatial highpass ?lter broadens the range of
`signals to produce a second filter output containing the target
`signals to produce a second ?lter output containing the target
`arrival angles by receiving multiple spatial samples from a
`arrival angles by receiving multiple spatial samples from a
`signal, the second spatial beamforming ?lter having a
`signal, the second spatial beamforming filter having a
`selected set of microphone outputs using a plurality of leaky
`selected set of microphone outputs using a plurality of leaky
`60 greater beam width than a beam width of the first spatial
`greater beam width than a beam width of the ?rst spatial
`adaptive ?lters.
`adaptive filters.
`beamforrning filter. The first adaptive filters are connected to
`beamforming ?lter. The ?rst adaptive ?lters are connected to
`However, a large number of microphones (the Q value)
`However, a large number of microphones (the Q value)
`the output of the second spatial beamforming ?lter, instead
`the output of the second spatial beamforrning filter, instead
`are required to implement a beamformer having a wide
`are required to implement a beamformer having a wide
`of to the output of the first-named spatial beamforming filter.
`of to the output of the ?rst-named spatial beamforming ?lter.
`range of rejection angles, for each group of spatial highpass
`range of rejection angles, for each group of spatial highpass
`BRIEF DESCRIPTION OF THE DRAWINGS
`BRIEF DESCRIPTION OF THE DRAWINGS
`filters in the filter bank. If a sufficient number of micro- 65
`?lters in the ?lter bank. If a sufficient number of micro
`65
`The present invention will be described in further detail
`phones is not provided. the degree of design freedom must
`phones is not provided, the degree of design freedom must
`The present invention will be described in further detail
`with reference to the accompanying drawings, in which:
`with reference to the accompanying drawings, in which:
`be sacrificed, resulting in a beamformer having a low noise
`be sacri?ced, resulting in a beamformer having a low noise
`
`RTL923_1023-0013
`
`

`
`5,627,799
`5 .627,799
`
`3
`3
`FIG. 1 is a block diagram of a prior art adaptive array
`FIG. 1 is a block diagram of a prior art adaptive array
`beamformer;
`beamfonner;
`FIG. 2 is a block diagram of the spatial highpass ?lter of
`FIG. 2 is a block diagram of the spatial highpass filter of
`the FIG. 1 prior art;
`the FIG. 1 prior art;
`FIG. 3 is a block diagram of the leaky adaptive ?lters of
`FIG. 3 is a block diagram of the leaky adaptive filters of
`the FIG. 1 prior art;
`the FIG. 1 prior art;
`FIG. 4 is a block diagram of an adaptive array beam(cid:173)
`FIG. 4 is a block diagram of an adaptive array beam
`fonner according to a first embodiment of the present
`former according to a ?rst embodiment of the present
`invention;
`invention;
`FIG. 5 is a block diagram of an adaptive array beam(cid:173)
`FIG. 5 is a block diagram of an adaptive array beam
`fonner according to a second embodiment of the present
`forrner according to a second embodiment of the present
`invention;
`invention;
`FIG. 6 is a block diagram of the nonn constraint adaptive
`FIG. 6 is a block diagram of the norm constraint adaptive
`filters of the second embodiment;
`?lters of the second embodiment;
`FIG. 7 is a block diagram of the constraint coefficient
`FIG. 7 is a block diagram of the constraint coe?icient
`generator used in FIG. 6;
`generator used in FIG. 6;
`FIG. 8 is a block diagram of an adaptive array beam(cid:173)
`FIG. 8 is a block diagram of an adaptive array beam
`fonner according to a third embodiment of the present
`forrner according to a third embodiment of the present
`invention;
`invention;
`FIG. 9 is a block diagram of the coefficient-constrained
`FIG. 9 is a block diagram of the coef?cient-constrained
`adaptive filters of the third embodiment;
`adaptive ?lters of the third embodiment;
`FIG. 10 is a graphic representation of the input/output
`FIG. 10 is a graphic representation of the input/output
`characteristic of the limiters of FIG. 9;
`characteristic of the limiters of FIG. 9;
`FIG. 11 is a block diagram of an adaptive array beam(cid:173)
`FIG. 11 is a block diagram of an adaptive array beam
`fonner according to a fourth embodiment of the present
`forrner according to a fourth embodiment of the present
`invention; and
`invention; and
`FIG. 12 is a block diagram of a modification of the present
`FIG. 12 is a block diagram of a modi?cation of the present
`invention.
`invention.
`
`DErAILED DESCRIPTION
`DEI‘AJLED DESCRIPTION
`Before proceeding with the detailed description of the
`Before proceeding with the detailed description of the
`present invention. it may provide helpful to provide an
`present invention. it may provide helpful to provide